Advances in nanotechnology make it possible to produce components with structure elements becoming smaller and smaller. Examining the components produced, for example integrated circuits, requires tools that can scan these structures without mechanical contact, such that the measurement data can be taken as a basis for deciding whether the components fulfil a predefined specification. The clock frequency of integrated circuits extends right into the GHz frequency range (e.g., greater than 4 GHz). Since the bandwidth of present-day detectors, for instance of secondary electron detectors, at 10 MHz to 100 MHz is far below the clock frequency of integrated circuits that are customary nowadays, microelectronic components are typically measured stroboscopically during operation. This requires short pulses of charged particles, preferably electron pulses, with low jitter. The pulse duration and jitter have a major influence on the detection bandwidth that can be achieved. Analyzing components of an integrated circuit that are in operation typically necessitates, e.g., pulse durations of less than 50 ps and jitter of less than 30 ps.
Short pulses are likewise required in a second field of use, the repair of photolithographic masks (referred to as: Focused Electron Beam Induced Processing (FEBIP)). In addition, it is necessary to precisely control the times in which the beam is on or off, and indeed the pulse duration and pulse period or the pulse repetition rate make it possible to set the ratio of adsorption of gas molecules at the surface of a photolithographic mask and chemical conversion of the adsorbed gas molecules under the influence of the particle beam.
Pulses of charged particles can be generated in a plurality of ways. US 2009/0026912 A1 describes, inter alia, generating pulses by irradiating the cathode of an electron source with electron pulses. The incorporation of additional components into the electron source leads to a complex apparatus for generating a pulsed electron beam.
US 2005/0253069 A1 reports on the direct generation of short pulses in an electron source (of a heated LaB6 crystal) with the aid of pulsed photoemission, i.e. by bombardment with ultrashort light pulses originating from a laser system. What is disadvantageous about this method is the low radiant intensity of the electron pulses generated, which is insufficient for many applications.
On account of the disadvantages mentioned, at the present time pulses of charged particles are preferably still generated by blanking a continuous particle beam. A beam blanker typically comprises a pair of deflection plates and a stop having an aperture, through which the particle beam has to pass in order to reach the sample arranged behind or beam-downstream. As a result of an electrical voltage being applied to the deflection plates, the charged particle beam is deflected and impinges on the stop instead of passing through the aperture of the stop. The shortest time in which a charged particle beam can be switched on and off depends on the length and width of the deflection plates, the distance between the latter, and also the diameter of the aperture and the distance between the deflection plates and the stop. Furthermore, the achievable pulse duration is determined by the rate of rise of the voltage and the magnitude of the voltage at the printed circuit boards in the final state.
In order to alleviate the problems of deflecting a charged particle beam with the aid of printed circuit boards, in particular the rate of rise of the voltage, the U.S. patent specification with No. 4,721,909 proposes a coaxial transmission line for deflecting an electron beam. In addition, a controlled detector (“gated detector”) is used for filtering undesired pulses.
The partly conflicting requirements explained above make it difficult to generate short pulses using a beam blanker comprising a single pair of printed circuit boards, particularly if a compact design of the beam blanker is additionally demanded.
The author A. Gopinath describes in Chapter 9. VII. 8 of the book “Beam Processing Technologies,” edited by S. G. Einspruch, S. S. Cohen and R. N. Singh (ISBN 1-48-320442), deflection structures having more than two printed circuit boards which can be incorporated into an electro-optical column of a scanning electron microscope. By way of example, the author elucidates a complex deflection structure comprising two spatially separated deflection plate systems, in which the particle beam circulates on a circle after passing through the first deflection plate system and the duty ratio is settable by a multi-electrode deflection element. The second deflection plate system brings the charged particle beam back to its original beam direction again.
US 2014/0103225 A1 describes a beam blanker embodied in the form of a resonant structure. The resonant structure is excited with two slightly different frequencies, which enable a settable deflection of a charged particle beam in a plane perpendicular to the beam direction.
US 2012/261586 A1 describes a beam blanker that uses a pair of printed circuit boards to deflect a charged particle beam. The electric field is generated in a resonant LC structure at a resonant frequency f at which the charged particle beam sweeps over the aperture of a stop twice during a period duration. Two beam blankers can be arranged successively in the beam direction and jointly use specific components of a beam blanker, such as the stop, for instance. The charged particle beam generates a Lissajous figure on a stop.
The resonant deflection structures of the last two documents cited significantly restrict the flexibility when setting the pulse durations and in particular the pulse repetition rate or the pulse period, since the resonance condition defines the frequency at which the resonant deflection structures can be operated.
The information provided above is merely to assist the reader in understanding the background of the invention. Some of the information provided above may not be prior art to the invention.
The present invention therefore addresses the problem of specifying a beam blanker and provides a method for blanking a charged particle beam which at least partly avoids the disadvantages mentioned above.